The spring used in a vehicle suspension system deforms to absorb an impact and springs back in the direction for recovering the deformation when an external force disappears, resulting in a reciprocal oscillation. In order to reduce the oscillatory energy of the spring, the vehicle suspension system is provided with a damper called a shock absorber. The damping force of the damper should be small in order to ease impact but should be of a certain large value in order to enhance the road holding ability and operational stability of the wheels. A variable damping force damper that can vary the damping force is known as a system that can satisfy such trade-off conditions (see Japanese Patent Application Laid-Open No. 60-113711, for example).
Meanwhile, so-called skyhook control is known as a way of damper control for improving riding comfort by using the variable damping force damper (see Japanese Patent Application Laid-Open No. 06-247121, for example). The basic idea of the skyhook control is generally explained below with respect to an exemplary situation where the wheel (or tire) runs over a projection on the road surface.
When the wheel runs onto the projection, the vehicle body moves upward and thus the sign of the vertical (or up-down) movement speed of the sprung mass is positive, while the damper undergoes contraction, resulting in a negative sign of its piston speed. When these two signs are opposite from each other like this, the damping force is controlled such that the damping force is reduced or the damper becomes softer.
Immediately after the wheel has passed over the top of the projection, the vehicle body continues to move upward due to inertia and thus the sign of the vertical speed of the sprung mass remains positive, but the damper is pulled up with the vehicle body and undergoes expansion, resulting in a positive sign of the piston speed. When the two signs are the same like this, the damping force is controlled such that the damping force is increased or the damper becomes stiffer.
When the wheel begins to move downward after running over the top of the projection, the damper is pulled downward by the unsprung mass and expands, thus the sign of the piston speed is kept positive. The vehicle body is also moved downward, and this makes the sign of the vertical movement speed of the sprung mass negative. Thus, the two signs are opposite from each other and the damping force is controlled such that the damping force is reduced or the damper becomes softer.
Then, immediately after the wheel has reached the flat road surface after completing running over the projection, the vehicle body continues to move downward due to inertia and thus the sign of the vertical movement speed of the sprung mass remains negative, while the damper undergoes contraction because the wheel has stopped downward movement, resulting in a negative sign of the piston speed. Thus, the two signs are the same and the damping force is controlled such that the damping force is increased or the damper becomes stiffer.
In the technique for carrying out skyhook control disclosed in JPA Laid-Open No. 06-247121, it should be particularly mentioned that a state quantity of the unsprung mass that changes in response to an input from the road surface or vehicle behavior is evaluated, and if it is determined from the state quantity that the vehicle is traveling on a road surface where the resonance frequency of the unsprung mass constitutes a main component of the oscillation frequency, the damping force is controlled with a weighting coefficient suited mainly for suppressing the oscillation of the unsprung mass.
In the technique disclosed in JPA Laid-Open No. 06-247121, because the damping force is controlled mainly for suppressing oscillation of the unsprung mass, the road holding ability of the wheels can be enhanced but it sometimes sacrifices the riding comfort.